(I think the title you meant was
"popen spawned process may not
write to stderr under windows")
The child is dying with IOError:
[Errno 22] Invalid argument
at the sys.stderr.flush() call.
Neither the docs for os.popen nor
the Linux man page for popen(3)
say that stderr is redirected, so
one would expect the handle to be
inherited; the IOError looks like
a bug.
Try using os.popen4 or
popen2.popen4 or -the recommended
choice- the subprocess module.
Using the latter, this is the
modified parent.py:
"""
import subprocess
cmd = 'python child.py'
p = subprocess.Popen(cmd,
stdout=subprocess.PIPE)
for line in p.stdout:
print ">>>", line,
print p.wait()
"""
and this is the output, as
expected:
"""
2:stderr
>>> 1:stdout
>>> 3:stdout
0
"""
Note the 2:stderr line lacking the
>>>, because it was printed
directly by the child process onto
the stderr handle inherited from
its parent.

the popen call does not redirect stderr.
If you do something like 2>null (windows) or 2>/dev/null (*nix) it will
_never_ get printed.
If you want to have stderr & stdout getting in via popen and thus stdout,
under *nix and windows you would do that:
command 2>&1
It is not popen to blame.
See this for reference:
http://netbsd.gw.com/cgi-bin/man-cgi?popen++NetBSD-current

pythonmeister: I never expected stderr to be redirected, just *all
stdout* to be captured. But...
gagenellina: you are completely right about the failure. Still, this
issue happened with a real world application written in C, and
redirecting manually stderr to :NUL: solved it unexpectedly. I did not
expect spawned process behaviour to differ when its stderr is being
redirected.

We've discussed this at the PyCon sprints, and here's the concensus:
os.popen inherits the parents stderr, and on Windows there is not an
existing valid stderr by default. So the parent should, to be
compatible with the Windows environment, create stderr or use popen2.
However, popen is deprecated. The best solution would be to use
subprocess module which is defined to do the right thing in this case.
popen is not.
Because popen is deprecated, we are going to leave this behavior and
documentation as it is.